1. Field of the Invention
The present invention relates to a heating/cooling unit to selectively either heat or cool a set of dies used with an injection molding machine, particularly to such a heating/cooling unit devised to render service with use of a medium such as oil for dies cooling or heating.
2. Description of the Related Art
Of late, a great variety of injection molding machines are undertaking in great numbers various jobs. Of these, some injection molding machines, each running with a set of dies applied at normal temperature are nowadays confronted with a problem that plastic melt injected into a cavity created inside the dies start to set midway in a fluidity cycle due to spontaneous cooling therein, whereby the products are subject to bearing on their surfaces streaks, namely weld marks in another term, thus resulting in undergoing qualitative degradation.
The occurrence of weld marks can be prevented by heating the portion of a set of dies where plastic melt runs therealong following an injection cycle. However, constantly heating a set of dies entails some change of die thermal distribution, with not only the molding conditions getting unstabilized but also the setting time and the time up to a product take-out cycle commonly incurring some increases. To avoid these problems, the conventional injection molding process goes with heating prior to injecting plastic melt into the cavity within the dies, and cooling the dies before their opening after plastic melt injection.
In the prior art, there was developed a die heating/cooling unit for practical use, which runs, introducing heated oil as a thermal medium into the cavity within the dies in a die heating cycle, and in a die cooling cycle, likewise bringing cooled oil into said cavity.
FIG. 1 is a block diagram of such a conventional heating/cooling unit as referred to above, wherein 1 is a reservoir to contain oil 2, and the oil 2 in the reservoir is sucked up by an oil pump 4 which is driven by a motor 3. A discharge port of the oil pump 4 is connected to a first port 6a of a flow direction changeover valve 6 via a first oil path pipe 5 while a second port 6b of the flow direction changeover valve 6 has a first oil return pipe 7 coupled thereto to flow back the oil 2 in the reservoir 1, wherein halfway along the first oil return pipe 7, there are provided a heat exchanger 8 to cool the oil 2 on the way to return to the reservoir 1, and an oil filter 9. A third port 6c of the flow direction changeover valve 6 is maintained in fluid communication with an inlet 13a of an in-die fluid path 13 formed inside a set of dies 12 via a second oil path pipe 20 and a third oil path pipe 21. Mounted on a third oil path pipe are not only a first check valve 15 to prevent a flow of the oil 2 from inside the die set 12 but also an oil heater 16 which is located midway between the first check valve 15 and the flow direction changeover valve 6 to heat the oil 2.
Further between a fourth port 6d of the flow direction changeover valve, and an inlet 13a of the in-die oil path 13, a fourth oil path pipe 17 is installed to provide fluid communication. The fourth oil path pipe 17 is connected to the third oil path pipe 21 and is further mounted with a second check valve 18 to prevent a back flow of the oil 2 from inside the die set 12.
An outlet 13b of the in-die fluid path 13 is coupled to an inlet of an oil cooler through a second oil return path pipe 19. The second oil return path pipe 19 to flow back the oil from inside the die set 12 into the reservoir 1 is coupled to the first oil return path pipe 7. A relief pipe 10 is provided which branches off from the first oil path pipe 5 and is connected to the first oil return path pipe 7. A relief valve 11 is installed on the relief pipe 11 to regulate the fluid pressure over a range of 10 through 20 kg/cm.sup.2 at the maximum. While the heating/cooling unit of such an arrangement as remarked above is maintained standing by, the oil 2 circulates in the direction of an arrow of a single dotted line in FIG. 1, getting cooled. More precisely with the mechanism of oil cooling, the motor 3 is driven with the flow direction changeover valve 6 held at a neutral position whereat the valve is kept deenergized, running the pump 4, whereby the oil 2 in the reservoir 1 is pumped up, flowing into the first oil path pipe 5. At this stage, the first oil path pipe 5 is blocked by the flow direction changeover valve 6 which is maintained at the neutral position, whereby along with the lapse of some time, the hydraulic pressure inside the first oil path pipe 5 goes up beyond a setpoint of the relief valve 11. Then, the oil 2 is forced to flow into the first oil return pipe 7 mounted with the heat exchanger 8 and the filter 9 through the relief pipe 10, thereby getting not only cooled by the heat exchanger 8 on the first oil return pipe 7 but also filtered with the filter 9 prior to the return to the reservoir 1. Therefore with the heating/cooling kept on, cooling of the oil 2 gets progressed.
In the process of heating the die set 12, the oil 12 is held circulating in the direction of an a solid line arrow, whereby the die set 12 is heated with heated oil. To be precise with the mechanism of heating the die set 12, driving the pump 4 with the right chamber of the flow direction changeover valve 6 energized concurs with sucking up cool oil 2 from the reservoir 1, with the oil 2 subsequently getting discharged from the third port 6c of the flow direction changeover valve 6 into the second oil path pipe 20, and then led into a third oil path pipe 21. Thus, the oil 2 is heated by an oil heater 16, and thereafter, the oil 2 flows into the in-die fluid path 13 via the check valve, transferring the energy of heat to the die set 12, whereby the die set 12 is heated up to a proper temperature where the occurrence of weld marks is not allowed.
In the meanwhile the oil 2 flowing out from an outlet 13b of the in-die oil path 13 is led into the first oil return pipe 7 by way of a second oil return pipe 19, whereby the oil gets not only cooled with the heat exchanger 8 but also filtered clean by the filter 9 prior to the return to the reservoir 1.
Further in the process of cooling the die set 12, the oil 2 is kept circulating in the direction of a broken line arrow in the figure, thereby the die set 12 is cooled with the oil 2 carrying coolness. Precisely with the mechanism of cooling the die set 12, driving the pump 4 with the left chamber of the flow direction changeover valve 6 energized coincides with pumping up cool oil 2 from the reservoir 1, subsequently discharging the oil into the fourth oil path 17 via the fourth port 6d, and then getting led into the in-die fluid path 13 via a second check valve 18, whereby the die set 12 is cooled. This results in quickly cooling a product molded within the cavity of the die set 12, following the injection of plastic melt, down to a temperature at which the product can be taken out with quickness.
Meanwhile the oil 2 flowing out from the outlet 13b of the in-die fluid path 13 is led into the first oil return pipe 7 via the second oil return pipe 19, whereby the oil 2 gets cooled with the heat exchanger 8 and then filtered clean with the filter 9 prior to the return to the reservoir 1.
Introducing a thermal medium into the in-die fluid path 13 inside the die set 12 as in the foregoing proved serviceable to selectively heat or cool the die set 12, whereby quality injection molding is assured with freedom from leaving behind welds in the surface of an molded product.
The conventional heating/cooling unit of a prior art is found failing to exhibit high efficiency over heating and cooling the oil 2, a thermal medium, with the result that much time is required to heat and cool the die set 12, and there comes up to the fore a problem over the failure to cut short the cycle time covering the process of injection molding and a job step of molded product take-out. It is further noted with the conventional heating/cooling unit that to undertake quick heating, the oil heater 16 with a large capacity is essential, and for quick cooling, the heat exchanger 8 owning a large capacity is required, and also that unless the pump 4 and the motor to drive thereof are provided each with a large capacity, the efficiency of oil heating and cooling worsen, resulting in impracticality to cut short the cycle time of an injection molding machine.
Analyzing such problems with the conventional heating/cooling unit and subsequent researches disclosed that the settlement of the following carries vital significance:
(1) With the conventional heating/cooling unit, the oil 2, a thermal medium is directly delivered to the oil heater 16 as cooled, with the efficiency of oil heating thus resulting in worsening, whereby it eaves behind an interference with high-speed oil heating over a very short span of time.
(2) Similarly to the case of heating referred to above, the oil 2 flowing out with a high temperature from the oil heater 16 is flowed back to the oil cooler 8 immediately after the passage through the in-die fluid path 13 so that the efficiency of oil cooling gets worse, whereby to make up for this, an oil cooler with a large capacity is required.
(3) Though energizing the motor 3 even with the flow direction changeover valve 6 kept at the neutral position drives the pump 4 thereby to suck up and discharge the oil 2, the oil 2 fails to circulate through a relief pipe 10 (a reservoir circuit) until the oil pressure is boosted up to a setpoint (10 through 20 kg/cm.sup.2) of the relief valve 11, which must remain unchanged. To force-circulate the oil through the relief pipe, the motor 3 must be driven, with some waste of electric energy as a result.
(4) Shifting the flow direction changeover valve 6 from one position to another for the process of oil cooling takes place flowing back the oil 2 thus far staying in the oil heater 16 directly into the oil cooler 8, thus giving rise to a problem in combination with a preceding problem remarked in (2) above.